Sound waves propagate very well in liquids, since liquids such as, for example, water generally have a low damping capacity. In the sea, sound waves caused by, for example, whales or underwater construction sites are carried over many kilometers. Sound waves, however, can be hazardous to the animals in the water, such as marine mammals or fish. This applies particularly to artificially generated sound waves. Pile driving gives rise to particularly powerful acoustic emissions in water.
In addition to increasing shipping traffic, the erection of offshore wind energy installations is one of the sources of acoustic emissions in water that will increase in the future and that impose substantial stresses upon animals. Measures to reduce the acoustic emissions and to adhere to allowable limit values are necessary in order that animals, in particular protected species such as, for example, porpoises and seals, are not adversely affected by technical installations in the sea, in harbor areas and in other bodies of water.
Acoustic insulation and acoustic damping are known for the purpose of reducing the propagation of sound and wave motions in water.
Acoustic insulation is the use of reflective obstacles to impede the propagation of sound. The strength of the reflection is dependent on the difference in the wave resistances of the sound-carrying medium and the impeding medium. This effect is particularly pronounced in the case of acoustic transmission from water to air and then back from air to water. The reduction of sound propagation in water using the principle of an acoustic insulating and shielding air layer is known from DE 103 02 219 A1. The method, disclosed in the document, for reducing the transmission of sound and wave motions in the case of an object in the water relates to realization of the air layer that is to fully surround the object through, for instance, tubes, air cushions, foam layers or porous or air-filled films. In each case, the sound source must be enveloped in a completely form-fit closed manner in order to achieve the hoped-for acoustic insulating effect. Even small openings, as in the case of a door gap or similar to structure-borne sound bridges, reduce the acoustic insulating effect considerably and render it ineffective. Depending on the design of the acoustic insulation, the acoustic insulating effect is greatly reduced in the resonant ranges of the device.
Acoustic insulation thus requires a complete enveloping of the sound source. In the establishing of relieved foundation structures or the driving of sheet piling in water, complete form-fit enveloping is achievable only with a very large amount of resource, but is usually not achievable. Particularly in the offshore domain, with the motion of the sea and great depth of water, the manipulation of acoustic insulation structures is resource-intensive. Only with difficulty can a complete decoupling, i.e. the placing in the water of a continuous, vertical air layer that completely surrounds the sound source, be realized with an economically justifiable resource expenditure, particularly in the case of water depths of over ten meters. Reasons for this are the water pressure that is present and the horizontal forces from the current. A decoupling, i.e. acoustic insulation, by means of air-filled hollow bodies, such as that described in the publication DE 103 02 219 A1, always has sound bridges and resonant ranges. Thus, the air layer at the connection points of the individual hollow bodies is extremely thin, or even discontinuous. Since the resonant frequency of the hollow bodies is often in the lower frequency ranges of less than 100 Hz, these frequencies, typically often generated in the case of drilling and pile-driving operations, are transmitted or even amplified by the oscillating hollow bodies.
Further devices for acoustic insulation are described by Dr. Manfred Schulz-von Glahn, Dr. Klaus Betke and Dr. Georg Nehls in their publication “Minderung des Unterwasserschalls bei Rammarbeiten für Offshore-WEA—Praktische Erprobung verschiedener Verfahren unter Offshore-Bedingungen”. Moreover, a sound scattering and damping method, the so-called bubble curtain, is also mentioned in the publication.
Acoustic damping, by contrast, is the absorption of the sound, i.e. the conversion of the mechanical acoustic energy into heat, whereby the acoustic energy is nullified. The publication U.S. Pat. No. 3,647,022 A discloses a device for damping sound waves in a liquid medium, wherein individual sound damper elements are arranged on a carrier element, a wall of a vessel that accommodates the liquid medium constituting the carrier element that absorbs the resulting sound pressure.
In the case of liquids, which as a medium have a low damping capacity, the sound can also be damped, for example, by the oscillations of a multiplicity of gas bubbles. The sound excitation in the range of the natural frequency of the individual gas bubble results in a very effective reduction of the sound amplitudes, both through scattering and through absorption of the sound. The natural frequency of a gas bubble in this case is dependent, inter alia, on the elasticity, the pressure and the diameter of the gas bubble.
Curtains of gas bubbles have already been applied successfully in pile-driving operations in shallow water. In addition to the resonant frequency, the rise velocity of the gas bubbles is also dependent on the diameter of the gas bubbles. In a curtain having a mixture of gas bubbles of differing diameters, the larger bubbles rise very much more quickly. The gas bubbles that rise up slowly must be shielded from current influences, by means of appropriate measures. For this, it is usual to employ a so-called guided bubble curtain, in which the curtain of gas bubbles rises up within a body, the body being impermeable to current and thus having to absorb the horizontal forces from the current acting upon it. A curtain of gas bubbles can be used to influence the hydroacoustic properties of the medium water. For this purpose, the gas bubbles are usually obtained from the ambient air present above the water level and are produced in the water, usually in a plurality of planes, by means of technical equipment such as pumps and lines.
Each gas bubble is released in the water and held together by the surface tension of the water. The transmission of the sound in this case is reduced, in essence, through damping, scattering and absorption. Such a curtain is produced by means of tubes and/or pipelines laid on the ground. The tubes and/or pipelines have openings, of a defined size and quantity, through which gas is forced into the surrounding water. Usually, the air present above the water level is used as the gas for the bubbles. This air is compressed by compressors and conveyed to the tubes and/or pipelines laid on the ground. The production of such curtains of bubbles of a defined size and quantity becomes more demanding as water depth increases, since the volume of the individual rising bubble is dependent on the water depth. As the gas bubble rises in the water, the water pressure surrounding the gas bubble decreases, which results in a considerable change in the size of the gas bubble, and therefore in the effective frequency range, and in uncontrolled conditions due to non-controllable divisions and combinations of the gas bubbles. Since the resonant frequency of the bubble varies with its volume, it is necessary for bubbles to be produced continuously at differing depths, for example every five meters, in order to achieve, to some extent, controlled conditions of acoustic damping.
The production of a curtain of bubbles for the purpose of damping sound emissions from industrial installations such as oil drills or pile-driving operations for wind power installations necessitates the compression and conveyance of large quantities of compressed gas. The equipment required for this has a high energy requirement and high operating costs, which increase as water depth increases. Sound emissions are transmitted, not only through the water, but also through the ground, and can then be emitted back into the water at a distance from the sound source. From an economic and ecological point of view, however, the production of a large-area curtain is questionable. The curtain is also adversely affected by the currents in the water, resulting in the bubbles being generated in an uncontrollable manner and, ultimately, in a less effective acoustic damping.